1. Field of the Invention
The present invention relates to an adjustment apparatus for a laser light source and a condensing optical element in a light source unit to be installed on an optical scanning apparatus including a deflector, and to an adjustment method for the adjustment apparatus. The present invention more particularly relates to an adjustment apparatus for and an adjustment method of adjusting a relative position between a laser light source and a condensing optical element of a light source unit. The light source unit includes at least the laser light source and the condensing optical element for condensing a light beam emitted from the laser light source and leading the light beam to a deflector, and the light beam emitted from the light source unit is restricted for use by a stop provided between the laser light source and the deflector in an optical scanning apparatus so that a beam width in a main scanning direction and a beam width in a sub-scanning direction are different from each other.
2. Description of the Related Art
Conventionally, in manufacture of a light source unit including a laser light source and a condensing optical element of an optical scanning apparatus installed on a laser beam printer and a digital copying machine, a relative position between the laser light source and the condensing optical element is finely adjusted in an independent process. A scanning optical system on a stage subsequent to the light source unit is usually a magnifying optical system in the optical scanning apparatus, and it is necessary to strictly adjust the relative position of the laser light source with respect to the condensing optical system of the light source unit. If the strict adjustment is not carried out, a beam spot diameter increases on a surface to be scanned such as a photosensitive drum, or a beam cannot be imaged on a desired irradiated position, resulting in difficulty in high-definition printing and color registration alignment.
Some proposals have been made for an adjustment method and an adjustment apparatus for a relative position between the laser light source and the condensing optical element of the light source unit. According to an adjustment tool described in Japanese Patent Application Laid-Open No. H08-136781, light emitted from an attached light source unit is imaged as a spot by a tool lens. For the imaged beam spot, a detection system constructed by an objective lens, a magnifying optical system and a camera reads a profile of the beam spot, a beam diameter is computed by an image processing device, a center of focal depth giving the minimum beam spot diameter is computed, and it is determined whether or not the irradiated position on a cross section orthogonal to an optical axis is at an optimal position. A lens barrel holding a condensing optical system is three-dimensionally controlled and driven so that a focus and the irradiated position are optimal. FIG. 20 summarizes this adjustment method as an algorithm. In a flowchart illustrated in FIG. 20, a collimator lens is moved at a pitch of dX in a range of ±dX×N with respect to a design position Xo in the focus direction for 2×N+1 times, the profile of the beam spot is read at each position, thereby computing a defocus amount ΔX, and further computing irradiated position shifts ΔY and ΔZ (paragraphs [0034] to [0038] and FIGS. 4A to 8 and FIG. 14 in Japanese Patent Application Laid-Open No. H08-136781).
According to Japanese Patent Application Laid-Open No. H08-136781, a determination method for the focus determination based not on the beam spot diameter but on a peak value of the quantity of light of the laser beam is also described (paragraph [0049]).
Moreover, an optical scanning apparatus of multi-beam type such as monolithic multi-beam type including the vertical cavity surface emitting laser (VCSEL) is made into product as the optical scanning apparatus. It is also necessary to align perpendicularity of a row of light emitting points with respect to an optical axis of a condensing optical system as a result of the employment of the multi-beam configuration. If the perpendicularity is not aligned, a difference in focus among beams, namely a so-called asymmetric blur is generated. For example, Japanese Patent Application Laid-Open No. H11-064755 proposes a method involving, for two light emitting points, after focusing for a first light emitting point, repeating, for some times, rotational adjustment of a laser element and fine adjustment of a collimator lens while observing focusing for a second light emitting point (paragraph [0024], FIGS. 4A and 4B).
However, the conventional adjustment methods for the laser light source unit have a problem that a long adjustment time is necessary.
According to Japanese Patent Application Laid-Open No. H08-136781, it is necessary to repeat a process of changing the interval between the laser light source and the condensing optical element by a small amount dX (defocusing), reading the profile of the beam spot, and computing the spot diameter and the peak quantity of light for a predetermined number of times. The reading of the spot profile and the computing of the spot diameter are repeated for 2×N+1 times in FIG. 20. A best focus position is computed based on a change in the spot diameter and the peak quantity of light according to the defocusing, thereby adjusting the interval between the laser light source and the condensing optical element, which thus poses a problem that man-hours are required for the repeated operation.
It is further necessary to repeat a process of defocusing a detection optical system including a camera, reading the profile of the beam spot, and computing the spot diameter and the peak quantity of light for a predetermined number of times in order to confirm the focus position after the interval between the laser light source and the condensing optical element is determined and fixed. This confirmation process also takes an adjusting time.
Moreover, according to Japanese Patent Application Laid-Open No. H11-064755, the number of focus measurements for each of the light beams, which need to be carried out according to the number of beams, increases in order to carry out adjustment of defocus for each of light emitting elements of multiple beams, so-called asymmetric blur adjustment, resulting in an increase in the adjustment time.
If the laser light source and the condensing optical element are relatively shifted in a direction vertical to the optical axis, an error in accuracy of a print position is generated, resulting in difficulty in color registration alignment. If a strict adjustment of the light source is not made around the optical axis particularly in a multi-beam scanning system, a beam pitch is not achieved at a desired interval, resulting in image degradation. As a result, an adjustment time increases for repeating measurement and adjustment of the print position of each of the light beams. Therefore, the strict positioning of the laser light source and the condensing optical element is necessary in the direction orthogonal to the optical axis, which causes a long assembly/adjustment time.